There is an atomic clock (an atomic oscillator) as a clock which measures time in an extremely precise manner and techniques, etc., for miniaturizing the atomic clock are being studied. The atomic clock is an oscillator which has, as a reference, a transition energy amount of electrons which make up an atom of an alkali metal, etc.; in particular, a very precise value is obtained for transition energy of the electrons in the atom of the alkali metal when there is no disturbance, making it possible to obtain a few orders of magnitude higher frequency stability than that with a crystal oscillator.
In such an atomic clock, there are a few types. Of these, a CPT (coherent population trapping)-type atomic clock has around three orders of magnitude higher frequency stability than that with related art crystal oscillators, and also an ultra compact size and an ultra low power consumption may be expected therewith (Non-patent documents 1 and 2).
As shown in FIG. 1, in the CPT-type atomic clock which includes a light source 910 such as a laser device, etc.; an alkali metal cell 940 in which the alkali metal is sealed; and a photo detector 950 for receiving a laser light passing through the alkali metal cell 940, the laser light is modulated and excited by sideband wavelengths which appear on both sides of a carrier wave being a specific wavelength causing two transitions of electrons in an alkali metal atom to be simultaneously conducted. The transition energy in this transition is invariant, and a clearing response occurs in which a light absorbance in the alkali metal decreases when the sideband wavelengths of the laser light match a wavelength corresponding to the transition energy. In this way, it is an atomic clock, wherein a wavelength of the carrier wave is adjusted such that the light absorbance by the alkali metal decreases; a signal detected in the photo detector 950 is fed back to a modulator 960; and a modulation frequency of the laser light from the light source 910 such as the laser device, etc., is adjusted by the modulator 960. The laser light is emitted by the light source 910 and irradiated to the alkali metal cell 940 via a collimating lens 920 and a λ/4 plate 930.
Methods are disclosed of producing an alkali metal cell in such an ultraminiature atomic clock using an MEMS (micro electro mechanical systems) technique (Patent Documents 1-4). In the methods disclosed therein, first an opening is formed on an Si substrate by a photolithography technique and an etching technique, after which a glass and the Si substrate are anodically bonded. The anodic bonding is carried out by applying a voltage of around 250 V to 1000 V to an interface between the glass and the Si substrate at a temperature of between 200° C. and 450° C. After putting in the alkali metal and a buffer gas, an opening portion to be a top face is sealed by anodically bonding the glass thereto. What is formed in this way is diced for each cell, so that the alkali metal cell is formed.
Methods of sealing the alkali metal within the cell include a method in which Cs (Cesium) metal is directly injected in a vacuum to be sealed (Non-patent Document 3); a method in which a solution in which CsCl is mixed with a BaN6 aqueous solution is injected in the cell and the Cs metal is produced by being reacted at 200° C. (Non-patent Document 3); a method in which the Cs metal is produced by reacting BaN6+CsCl in an ampoule with a heater and evaporated and transferred into the cell (Non-patent Document 4); a method in which, with a common evaporation scheme, a film of CsN3 is formed in the cell and the cell is sealed, after which a UV light is irradiated thereon to produce Cs and N2 (Non-patent Document 5); and a method in which, after a Cs dispenser, which is stable in atmosphere, is injected into the cell and the cell is sealed, a laser light is irradiated only onto the Cs dispenser to heat it, causing Cs to be produced (Non-patent Document 6).